1. Field of Invention
The present invention relates generally to compensation for gate leakage currents in a current mirror and specifically to compensation for gate leakage currents in a current mirror of a digital to analog converter (DAC).
2. Related Art
A digital-to-analog converter (DAC) is an electronic circuit that converts a digital input signal from a digital representation in the digital signaling domain to an analog representation in the analog signaling domain to provide an analog output signal. DACs can be designed for a wide range of applications, including general data acquisition applications and special applications, such as, but not limited to, video or graphic outputs, high definition video displays, ultra high-speed signal processing, and digital video recording.
A current-steering DAC represents a type of DAC that performs this conversion in a current domain. For example, the current-steering DAC includes a plurality of current sources, typically one for each bit of the digital input signal, that are arranged in parallel. The plurality of current sources are steered in accordance with the digital input signal to provide an analog representation of the digital input signal to as the analog output signal. Those current sources that are steered in a first direction contribute their corresponding currents to the analog output signal while those current sources that are steered in a second direction withdraw their corresponding currents from the analog output signal.
Ideally, transistors that form the current sources have sufficient insulation, commonly in the form of a gate oxide layer, between their respective gates and their respective drain to source channels such that no current flows from their respective gates to their respective drain to source channels during operation. However, the continuous down-scaling of these transistors has led to very thin oxide layers; thereby, allowing unwanted currents, referred to as gate leakage currents, to flow from their respective gates to their respective drain to source channels. Typically, these gate leakage currents increase exponentially as thicknesses of the gate oxide layers are reduced. Additionally, these leakage currents may vary in a large range over process and temperature. This variation commonly translates into a change of the full scale output of a conventional current-steering DAC which is an undesirable effect in communications systems where transmitted power is accurately specified, such as asymmetric digital subscriber line (ADSL) to provide an example.
Thus, there is a need for a method and an apparatus to compensate for the gate leakage currents that are associated with very thin oxide layers as described above. Further aspects and advantages of the present invention will become apparent from the detailed description that follows.
The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number.